The present invention relates to a diesel engine with a low compression ratio in which its geometric compression ratio is set comparatively low.
In diesel engines mounted in automobiles, a fuel injection is performed in each of cylinders for a plurality of times during one cycle of the engine in order to, for example, reduce NOx and soot contained in exhaust gas, reduce noises or vibrations, and improve a fuel consumption and torque. For example, JP2009-293383A discloses a diesel engine with a turbocharger in which fuel injection is performed at five timings: a main injection for generating a torque, a pilot injection performed prior to the main injection so as to preheat cylinders, a pre-injection performed between the pilot injection and the main injection to suppress an ignition delay of fuel injected by the main injection, an after injection performed after the main injection so as to raise a temperature of exhaust gas, and a post injection for raising a temperature of a catalyst by directly introducing the fuel to an exhaust system subsequent to the after injection.
Meanwhile, in a diesel engine, after fuel is injected, a time lag (i.e., an ignition delay) exists before a combustion starts. When the ignition delay is long, a slope of a heat release rate (=dQ/dθ in which “Q” indicates an amount of heat and “θ” indicates a crank angle) of the combustion becomes steep and a combustion noise become louder, and thereby, an NVH performance degrades. Therefore, the ignition delay is desired to be comparatively shorter in the diesel engine. However, in an engine with a low compression ratio in which its geometric compression ratio is set to be, for example, 15:1 or lower, a temperature and a pressure are low at the end of a compression stroke, and therefore, the ignition delay tends to be extended.